Parser/Python.asdl may need changes to match the Grammar. Run
make to regenerate Include/Python-ast.h and
Python/Python-ast.c.

Python/ast.c will need changes to create the AST objects
involved with the Grammar change. Lib/compiler/ast.py will
need matching changes to the pure-python AST objects.

Parser/pgen needs to be rerun to regenerate Include/graminit.h
and Python/graminit.c. (make should handle this for you.)

Python/symbtable.c: This handles the symbol collection pass
that happens immediately before the compilation pass.

Python/compile.c: You will need to create or modify the
compiler_* functions to generate opcodes for your productions.

You may need to regenerate Lib/symbol.py and/or Lib/token.py
and/or Lib/keyword.py.

The parser module. Add some of your new syntax to test_parser,
bang on Modules/parsermodule.c until it passes.

Add some usage of your new syntax to test_grammar.py

The compiler package. A good test is to compile the standard
library and test suite with the compiler package and then check
it runs. Note that this only needs to be done in Python 2.x.

If you've gone so far as to change the token structure of
Python, then the Lib/tokenizer.py library module will need to
be changed.

Certain changes may require tweaks to the library module
pyclbr.

Documentation must be written!

After everything's been checked in, you're likely to see a new
change to Python/Python-ast.c. This is because this
(generated) file contains the SVN version of the source from
which it was generated. There's no way to avoid this; you just
have to submit this file separately.